The claims about Felix are because someone noticed he had kestrel feathers in his mouth during defeathering the other day...
I would initially take that with a big grain of salt. Individual feathers are notoriously difficult to assign to a particular species of bird. Our birds eat shorebirds and there is no way on camera that we could tell one shorebird species' feathers from another shorebird species' feathers unless it had a yo-honking big purple spot on it. Besides, European Kestrels are not small birds - 32-39cm (body length) whereas peregrines are 34-50cm. Peregrine wingspan is greater and they weigh more however. So while not impossible, keep in mind that the kestrel is as capable a hunter/defender/protector as the peregrine, better some would say because they are smaller and quicker in the air at close quarters. The peregrines would risk serious injury to themselves going after the kestrel (and vice versa) something no predator can afford and their is no real profit in it - the kestrel/peregrines are not interested in each others chicks, they tend to hunt different prey (bigger bird = bigger prey items = more food) so not rivals in the strictest sense ...
I'd look for something much more mundane, collision with a building, a vehicle, high-tension wire ....
